Vision correction and magnification lenses for eyeglasses were originally made of glass. More recently, lenses have been made from polycarbonate material molded in different types of injection molding processes. Polycarbonate lenses are much lighter than glass lenses of the same power, and are sufficiently strong to pass a standardized drop/breakage test, ANSI Z80.1-1995, in which a 0.56 ounce steel ball is dropped from a vertical height of 50 inches on to the center of the lens, even with a relatively thin center thickness in a range of approximately 1.0 to 1.5 mm. However, polycarbonate lenses have the disadvantages of producing birefringence at the perimeter of the lens, and a tendency to yellow with age.
Acrylic material, also referred to as polymethylmethacrylate or PMMA, is a preferred material for optical lenses due to its superior optical properties, light transmittance, reduced weight, and lack of tendency to yellow with age. However, molded acrylic is more brittle than molded polycarbonate, due to its relatively low glass transition temperature, higher density and higher tensile strength. Although magnifying, non-refractive, and positive diopter ophthalmic lenses have been molded out of acrylic, such lenses generally have molded-in stresses, produced by shearing of the material in the molding process, which make them brittle and not able to pass the standard drop ball impact test. Shearing of the acrylic material is also caused by the wide range of thickness across the lens, particularly in the high-minus ophthalmic lenses, which may range from a center thickness of one millimeter to an edge thickness as much as ten millimeters. Discontinuity in the rate of flow of material into the mold cavity also causes shearing which produces birefringence in the molded lens.
In a prescription lens mold cavity, the molten acrylic tends to flow in a generally annular path about the thickest cross-sectional areas of the mold first, and then fills in the thinnest area, for example, at the relatively thin center region of a minus power lens. If not injected into the mold under the correct temperature, pressure and velocity, a weld line will form where the annular flows of material converge, at a point generally opposite the mold gate. Because of the extreme difference in cross-sectional thickness of the mold cavity, the molding material will cool at different rates, resulting in "sinks" or the solidification of the thinner portions before solidification of the thicker portions, also producing molded-in stresses which make the lens brittle and unable to pass the standard impact drop test. Also, molding lenses under relatively high pressures, up to 20,000 psi plastic pressure, produces greater stresses and birefringence in the lens as compared to lenses formed under lower pressure.
Many different approaches have been taken to the injection molding of lenses. In addition to straight injection (which generally produces unsatisfactory results), injection-compression processes, using variable clamp pressures on the movable half of the mold, and insert mold assemblies have been used. Also, in traditional injection-compression processes the halves of the mold are separated to a visible extent, e.g. up to 2 or 3 mm during the filling phase, making the rate of cavity fill critical to avoid blow-out of material between the mold halves. This type of consistent close process control is made more difficult when using acrylic because of its relatively low glass transition temperature, and for this reason it has been avoided as a material for lenses with high thickness variation.
With insert mold assemblies, there is typically some type of movable insert within one or both of the mold halves, which are movable relative to the mold cavity, to change the volume of the cavity during the molding process. In some systems, movement of the inserts is controllable independent of the clamp force. Insert control mechanisms of the prior art are typically complex and duplicated for each cavity of the mold, making them expensive and difficult to maintain. Also, the movement of the inserts must be controlled precisely in accordance with the introduction of material into the mold cavity in order to achieve the desired optically clear results.